Natural gas deposits are exploited by means of a production well. The gas extracted from the ground in production zones is then transported, e.g. by means of pipelines, to utilization zones, or else large quantities of gas may be stored underground again in the form of underground layers of gas.
Access is obtained to layers of gas stored underground likewise by means of a well of conventional configuration. It is often necessary to take action in production wells giving access to deposits of gas or in service wells subjected to the pressure of supply of gas stored underground.
For this purpose, a pressure lock is disposed by lifting tackle on top of the blow-out preventer (BOP) situated on the well head, after a wireline tool has previously been installed in the pressure lock. After the pressure lock has been connected to the BOP, the well head operating valve is opened in order to equalize the pressure in the pressure lock (which was initially at atmospheric pressure) and in the well (which may be subjected to pressure of the order of 10.sup.7 to 2.10.sup.7 pascals). The tool is then lowered down the well by means of a winch and the cable which is connected to the tool. After the action has been performed by the tool, it is raised back into the pressure lock and the well head operating valve is closed.
Thereafter, the gas contained in the pressure lock and the wire closure valve of the BOP needs to be bled off before the pressure lock can be disconnected from the BOP.
The operation of bleeding the gas present under pressure in the pressure lock is normally performed by means of a small, manually controlled bleed valve situated at the bottom end of the pressure lock above the BOP. However, this method of proceeding suffers from several drawbacks. In particular, the operator must climb up scaffolding in order to be able to open the bleed valve which is not directly accessible from the ground even though it is situated at the bottom end of the pressure lock. There is thus a risk of the operator falling. In addition, when the valve is opened, the operator is immersed in gas and is directly subjected to the aggression of the considerable noise generated when the valve is opened by virtue of the fact that the gas is throttled. Insofar as the operator opens the valve progressively in order to control the speed of gas escape, he must remain on the scaffolding and be subjected to the noise and the gas aggression throughout the time required for bleeding.
Since the manual bleed valve is situated at a height of about 2 meters above the ground, it may give rise to a very low altitude cloud of gas being formed, thereby running a risk of explosion since the pressure lock is normally hoisted by means of a crane fixed to a truck located in the vicinity of the well head, with the internal combustion engine of the truck running in order to provide the hydraulic power required by the crane.
The present invention seeks to remedy the above-mentioned drawbacks and to enable a well head pressure lock to be bled in a convenient manner while also providing a high degree of safety for the personnel concerned.